"Not only some, but most, of the fingerprint examiners changed their minds," said Dror, who was far less surprised by the flip-flopping. As an expert in human thought processes and the hidden power of cognitive bias — an array of distortions in the way humans perceive reality — he had a decided advantage.

Fingerprints have been accepted as unassailable evidence in courts for more than 100 years, but vaunted claims of their uniqueness and infallibility still lack scientific validation. By contrast, the existence of cognitive bias and the subjective nature of human judgment have been thoroughly established by hundreds of scientific studies going back decades.

. . .

Still, television's CSI this is not. Despite all that technology, it then falls to fallible human beings to step in and make visual comparisons and the ultimate judgment calls on matches.

Although forensics experts routinely claim otherwise, "You don't have to be an expert in cognitive neuroscience to know that this kind of interpretation, evaluation and judgment of visual pattern is definitely not objective," Dror said.

The human factor opens the door to all kinds of contextual influences and biases because, he explained, "many times, when we process visual information subjectively, it depends on the context and who we are, what we expect and a whole variety of basic, well-established psychological and cognitive phenomena. And somebody saying that human judgment and perception is totally objective is like saying the Earth is flat. You can't believe it when you hear that.

. . .

Bias is a complex foe. An array of unseen influences may impact evidence results by affecting what examiners believe they see and diminishing their objectivity. And there are numerous cognitive and psychological biases to recognize and counter. As long as forensic examiners don't make determinations about pattern matches "blind," for example, they will be more vulnerable to what's known as "confirmation bias." Confirmation bias is where evidence is cherry-picked to emphasize what confirms preconceived ideas and what someone hopes or expects to see, and downplay what doesn't.

. . . "Conformity bias" — when examiners' judgments and perceptions are influenced by others' opinions or by peer pressure — is also common. "Authority bias," another pothole, is where decisions may be colored by a superior offering an opinion and maybe exerting subtle pressure to concur. Experts also may get locked in to a line of thought, and that can bias an outcome — so can momentum or feeling pressure to solve a case or having an emotional reaction to gruesome crime scene images.

. . .

A key National Academy of Science report recommendation — to move crime labs out from under law enforcement's wing and create a new national institute of forensic sciences — would surely help impartiality. If lack of funding delays that, "so be it," Dror said. "But you can't have it both ways. If there's no reform, don't say, 'I am 100 percent objective, I make no mistakes, there is no problem.'"

. . . In the interim, some steps can be taken. When further examiners are called on to verify the work of a first, they should always examine the evidence independently without knowing the earlier results.

Efficiency, scientific validity and objectivity could also be dramatically improved for a relatively small financial outlay by establishing and enforcing "best practices" in crime labs (another NAS report recommendation.) Best practices are formally documented standard operating procedures, processes and rules for how to do your work that are specifically designed to make it effective and efficient, and avoid error. Having best practices that all fingerprint examiners everywhere must adhere to would be a big step forward, Dror believes, but only if they are science-based and validated by experts in cognitive neuroscience, psychology and thought processes.